1. Field of the Invention
The present invention relates generally to operational amplifiers and, in particular, to a simple, effective technique for reducing the transconductance of the differential stage of an operational amplifier to increase bandwidth and/or phase margin.
2. Discussion of the Related Art
R. J. Widlar and Mineo Yamatake, "A Monolithic Power Op Amp", IEEE Journal of Solid State Circuits, Vol. 23, no. 2, pp. 527-535, April 1988, disclose a power amplifier integrated circuit (IC) wherein the lateral pnp transistors in the differential stage have their collectors split into two equal segments. The current mirror is fed from one collector on each transistor, making its operating current independent of the input voltage. This takes the phase shift of the current mirror out of the signal path.
By way of explanation, the current mirror's current is in fact a DC current; the signal incurs no delay in the current mirror for the precise reason that no signal is applied to the mirror. As a result, the total transconductance of the stage is decreased. As a further result, the total phase shift of the transconductance stage consists only of the delay introduced by the primary differential pnp pair, thereby improving the phase margin.